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Abstract

It is well documented that the majority of Tuberculosis (TB) cases diagnosed in
Canada are related to foreign-bom persons from TB high-burden countries. The Canadian
seasonal agricultural workers program (SAWP) operating with Mexico allows migrant
workers to enter the country with a temporary work permit for up to 8 months. Preiimnigration
screening of these workers by both clinical examination and chest X-ray
(CXR) reduces the risk of introducing cases of active pulmonary TB to Canada, but
screening for latent TB (LTBI) is not routinely done.
Studies carried out in industrialized nations with high immigration from TBendemic
countries provide data of lifetime LTBI reactivation of around 10% but little is
known about reactivation rates within TB-endemic countries where new infections (or reinfections)
may be impossible to distinguish from reactivation. Migrant populations like
the SAWP workers who spend considerable amounts of time in both Canada and TBendemic
rural areas in Mexico are a unique population in terms of TB epidemiology.
However, to our knowledge no studies have been undertaken to explore either the
existence of LTBI among Mexican workers, the probability of reactivation or the
workers' exposure to TB cases while back in their communities before returning the
following season. Being aware of their LTBI status may help workers to exercise healthy
behaviours to avoid TB reactivation and therefore continue to access the SAWP.
In order to assess the prevalence of LTBI and associated risk factors among
Mexican migrant workers a preliminary cross sectional study was designed to involve a
convenience sample of the Niagara Region's Mexican workers in 2007. Research ethics
clearance was granted by Brock University. Individual questionnaires were administered to collect socio-demographic and TB-related epidemiological data as well as TB
knowledge and awareness levels. Cellular immunity to M tuberculosis was assessed by
both an Interferon-y release assay (lGRA), QuantiFERON -TB Gold In-Tube (QFf™)
and by the tuberculin skin test (TSn using Mantoux.
A total of 82 Mexican workers (out of 125 invited) completed the study. Most
participants were male (80%) and their age ranged from 22 to 65 years (mean 38.5). The
prevalence of LTBI was 34% using TST and 18% using QFTTM. As previously reported,
TST (using ~lOmm cut-off) showed a sensitivity of 93.3% and a specificity of 79.1 %.
These findings at the moment cannot predict the probability of progression to active TB;
only longitudinal cohort studies of this population can ascertain this outcome. However,
based on recent publications, lORA positive individuals may have up to 14% probability
of reactivation within the next two years.
Although according to the SA WP guidelines, all workers undergo TB screening
before entering or re-entering Canada, CXR examination requirements showed to be
inconsistent for this population: whereas 100% of the workers coming to Canada for the
first time reported having the procedure done, only 31 % of returning participants reported
having had a CXR in the past year. None of the participants reported ever having a CXR
compatible with TB which was consistent with the fact that none had ever been
diagnosed with active pulmonary TB and with only 3.6% reporting close contact with a
person with active TB in their lifetime.
Although Mexico reports that 99% of popUlation is fully immunized against TB
within the first year of age, only 85.3% of participants reported receiving BOC vaccine in
childhood. Conversely, even when TST is not part of the routine TB screening in endemic countries, a suqDrisingly high 25.6% reported receiving a TST in the past. In
regards to TB knowledge and awareness, 74% of the studied population had previous
knowledge about (active) TB, 42% correctly identified active TB symptomatology, 4.8%
identified the correct route of transmission, 4.8% knew about the existence of LTBI,
3.6% knew that latent TB could reactivate and 48% recognized TB as treatable and
curable.
Of all variables explored as potential risk factors for LTBI, age was the only one
which showed statistical significance. Significant associations could not be proven for
other known variables (such as sex, TB contact, history of TB) probably because of the
small sample size and the homogeneity of the sample. Screening for LTBI by TST (high
sensitivity) followed by confirmation with QFT''"'^ (high specificity) suggests to be a good
strategy especially for immigrants from TB high-burden countries. After educational
sessions, workers positive for LTBI gained greater knowledge about the signs and
symptoms of TB reactivation as well as the risk factors commonly associated with
reactivation. Additionally, they were more likely to attend their annual health check up
and request a CXR exam to monitor for TB reactivation.